Handsfree or loudspeaking telephone devices are wellknown. Such devices generally use two transducers, one as the speaker and the other as the microphone. The two transducers must be acoustically decoupled to prevent positive feedback or singing. In the usual case, voice switching in one direction is controlled by the stronger signal with signals in the weaker direction being attenuated to minimize and reduce their effects.
An example of a similar system for a handsfree telephone is shown by U.S. Pat. No. 3,725,585, issued Apr. 3, 1973, of which I am a joint inventor. In that patent, signals at the voice frequency level from each direction are compared within an operational amplifier to cause the stronger signal to be amplified and the weaker signal attenuated. Two transducers are used, acoustically decoupled.
Call announcing key systems are a recent innovation in the field of telephony. Their advantages over previous key system designs lie in the speed and efficiency with which calls can be handled within a group. In the usual system, a connection from an attendant to a called station within a key group is provided only for one-way paging or announcing. That is, an attendant announced to the called party that he was wanted in a meeting or that he had an outside call waiting, but the attendant did not know whether anyone was at the location to hear the announcement unless some definite, proper response resulted.